Conventionally, in illumination apparatus of this type, a plurality of secondary light sources are formed by for example a rod-type optical integrator, and uniform illumination of a mask on which is described a circuit pattern is performed by utilizing the illumination from these secondary light sources. The illuminated circuit pattern is projected onto and exposes a wafer coated with photosensitive material, through a projection lens.
Recently also, attention has been attracted by a technique for improving the resolving power and depth of focus of the optical projection system of the exposure apparatus by altering the pupil shape of the illumination light by changing the shape of aperture stops which are arranged in conjugate positions of the two-dimensional light source, in accordance with the shape of the pattern that is projected onto the wafer.
In an illumination device as above, it is generally considered that the illumination distribution on the mask i.e. the illumination distribution on the wafer when no pattern is formed on the mask should be in a uniform condition; however, due to various causes such as contamination or eccentricity of the optical system or unevenness of the anti-reflective coating, if non-uniformity of the illumination distribution on the wafer is found, more precise exposure can be achieved by adjusting the illumination distribution so as to compensate for this. Furthermore, in some cases, better results are obtained by deliberately providing a distribution of the illumination on the wafer, depending on the shape of the pattern on the mask, and/or aberration of the projection lens, or the pupil shape of the illuminating light etc.
Likewise, although it is generally considered that the pupil shape of the illuminating light should be the same shape for all image heights on the wafer, sometimes better results are obtained by deliberately arranging for different pupil shapes for different image heights.